The present invention relates to methods of making sintered metal oxide articles having desired mechanical properties and interconnected pore structures, and oxide articles thereby produced.
Sintering of inorganic powder compacts into useful solid products is a common and efficient way of fabricating metals, ceramics, and cermets. The general pattern of ceramic sintering includes three stagesxe2x80x94initial, intermediate, and final. In the initial stage, the pore shape may vary greatly depending on the size and geometry of particle contacts, and the pore structure is open and fully interconnected. In the intermediate stage, where the porosity typically shrinks to 40-10%, the pores become smoother and typically have a cylindrical structure that remains interconnected. The open pore network becomes unstable when the porosity is reduced to about 8%; then, the cylindrical pores collapse into spherical pores, which become closed and isolated. The appearance of isolated pores manifests the beginning of the final stage of sintering, leading to the densest products.
Major efforts in ceramic sintering have been made to obtain advanced materials such as electronic ceramics, structural ceramics, and high toughness composites where desired properties are sought to be reached at maximal densification (minimal porosity). The use of ceramic materials that have been sintered through only the intermediate sintering stage, however, has been more limited. One such use of these materials is in the filtration of gases and liquids. Among ceramic metal oxides, filter materials which have been obtained are commonly made of alumina (Al2O3), zirconia (ZrO2), and aluminum-silicates.
The intrinsic properties of iron oxides, hematite (xcex1-Fe2O3) and magnetite (Fe3O4), make them well-suited for diverse applications. These oxides are among the least expensive and naturally abundant substances. They are refractory ceramic materials that are chemically stable in various gas and liquid media, hematite being particularly appropriate for use in corrosive and oxidative environments. Furthermore, hematite and magnetite are environmentally benign, which make them suitable for water filtration and various applications in food, wine, pharmaceutical and other industries where environmental and health requirements are paramount. Moreover, hematite is electrically non-conductive and non-magnetic, and magnetite is highly conductive and magnetic, so the two iron oxides cover a wide spectrum of desirable electric and magnetic properties.
There exist numerous methods to prepare hematite and magnetite powders to be used as powders in various applications. However, there is a need in efficient and practical (economical) processes of making mechanically strong hematite and magnetite articles by sintering the respective powders, particularly into filter materials. U.S. Pat. No. 3,984,229 discusses attempts to briquette iron oxide raw materials at elevated temperatures of 800xc2x0 C. to 1100xc2x0 C. and concludes that it has been impossible to find a sufficiently strong material for the briquette molds (col. 1, lines 60-68). U.S. Pat. No. 5,512,195 describes efficient transformation of hematite powder into a magnetite single phase by mixing hematite powder with various organic substances, serving as a binder and reducing agent, and sintering at 1200xc2x0 C. to 1450xc2x0 C. in an inert gas. The strength of the sintered magnetite phase and its pore structure have not been characterized.
To obtain strong sintered articles, high pressure is conventionally employed. For example, U.S. Pat. No. 4,019,239 describes manufacturing magnetite articles by sintering and hot compacting magnetite powder in air at 900xc2x0 C. to 1300xc2x0 C. and a pressure of 100 to 600 MPa (1000 to 6000 atm), leading to a dense body with a porosity less than 3%.
In addition to high pressure requirements, conventional sintering of metal oxide powders usually requires binders and other extraneous agents to shape a powder preform and obtain the desirable composition. For example, in U.S. Pat. No. 5,512,195, sintering of hematite powder to a magnetite single phase requires mixing hematite powder with various organic substances that serve as binders and reducing agents. By contrast, the sintering of hematite powder without incorporation of any organic substance at 1200xc2x0 C. to 1450xc2x0 C. in an inert gas makes the hematite-magnetite conversion so low that the process is unfit for industrialization. It would be highly desirable to develop an effective and economical sintering process of iron oxides without the use of any additives or high pressures.
In one aspect, the present invention includes a method of making metal oxide articles, and preferably iron oxide articles. The method comprises the steps of slightly pressing powder to a compact, the powder consisting essentially of a first oxide of the metal; and subjecting the compact to a heat treatment that causes the powder to sinter into a unitary body and results in the transformation of at least a portion of the first oxide to a second oxide of the metal. The powder comprises a first oxide that is substantially free from additives, at least a portion of which is transformed to a second oxide by oxidation or deoxidation during the method of the present invention. The method optionally includes one or more heating/cooling steps during the heat treatment process, resulting in additional oxidation/deoxidation cycles.
In another aspect, the invention includes sintered metal oxide articles, and preferably iron oxide articles, made by the method of the invention.
One advantage of the present invention is that it provides sintered metal oxide articles, and preferably iron oxide articles, of high mechanical strength and other desired mechanical properties.
Another advantage of the invention is that it provides sintered metal oxide articles, and preferably iron oxide articles, having interconnected pore structures capable of efficient filtering gases and liquids.
Yet another advantage of the invention is that it provides efficient and economical processes of making sintered metal oxide articles, and preferably iron oxide articles, without the need for sintering additives of any kind and/or high pressures.
The present invention provides sintered metal oxide articles, and preferably iron oxide articles, of desired mechanical properties, such as high strength, and an interconnected pore structure capable of efficient filtering of gases and liquids. In accordance with the invention, metal oxide powder is subjected to a heat treatment to transform at least a portion of the oxide into a different oxide. The heat treatment is conducted at temperatures less than the melting points of the oxides and for suitable holding times to sinter the powder into a unitary oxide article. The powders used in the present invention are said to consist essentially of metal oxide in that such powders are substantially free from other compounds and additives such as binders, reducing agents, and the like.
Heating regimes for sintering are chosen to cause the oxidation and/or deoxidation of the oxide such that it is transformed to a different oxide, with several oxidation/deoxidation cycles possible. Although not wishing to be bound by theory, it is believed that oxygen transport during deoxidation and/or oxidation contributes to effective sintering and the resulting desired mechanical properties and uniformity in appearance and interconnected pore structure of the sintered article body. The invention thus obviates the need for sintering additives and high sintering pressures.
The present invention is described with specific reference to iron oxide articles, and specifically iron oxide filters, that are made by sintering iron oxide powders. The scope of the invention, however, includes articles of other metal oxide materials, of any form and intended use, that are made by sintering metal oxide powders that undergo oxidation and/or deoxidation during the sintering process.
In cited embodiments, sintered iron oxide filters are produced in accordance with the present invention. For example, in one embodiment, hematite (xcex1-Fe2O3) filters are made from magnetite (Fe3O4) powder. In another embodiment, hematite filters are made from hematite powder, which transforms to magnetite and back to hematite during sintering. In yet another embodiment, magnetite filters are made from hematite powder.
The filters are made in a sintering process wherein metal oxide powder is placed into a mold and hand-pressed into a compact and subjected to a suitable heat treatment to cause sintering into a unitary body and oxidative/deoxidative transformation of the powder. Preferred molds are alumina rings typically having an inner diameter of from about 10 to about 70 mm, and a height of from about 3 to about 60 mm. The powder particles are of any suitable size for sintering such as, for example, about 50 to about 200 microns. Such powders are readily available.
The heat treatment of the invention is selected based on the thermal properties of iron oxides. In air at atmospheric pressure, hematite is stable at elevated temperatures up to about 1350xc2x0 C. but decomposes to magnetite at higher temperatures up to about 1450xc2x0 C. Because magnetite begins to decompose to wustite FeO at temperatures above 1450xc2x0 C., this represents an upper limit of sintering temperatures in atmospheric air. For subatmospheric pressures, suitable sintering temperatures are lower according to the pressure within a vacuum furnace. In cited embodiments of the present invention, vacuum sintering typically occurs at a pressure within the range of about 10xe2x88x924 to about 10xe2x88x925 torr, wherein hematite begins to decompose to magnetite at about 750xc2x0 C. and magnetite begins to melt at about 1300xc2x0 C. The process is optimized on the premise that higher sintering temperatures allow for shorter sintering times. At pressures of about 10xe2x88x924 to about 10xe2x88x925 torr, efficient sintering occurs at 950xc2x0 C. to 1250xc2x0 C., preferably at 1000xc2x0 C. to 1250xc2x0 C., and more preferably at 1150xc2x0 C. to 1200xc2x0 C.